1. Field of the Invention
The present invention pertains generally to the field of measurement of water flowing in partially and completely full pipes using a sensor that is not in contact with the flow, more particularly, to the class of devices that utilize ultrasonic energy to determine the channel velocity.
2. Background of the Invention
There are many cases where it is important to measure the rate of flow in a pipe. For example, one may need to know the flow rate to determine a billing rate between two communities, to assess the rate at which rain or ground water is entering the sewage system, to design a system expansion, or to control the rate at which a holding tank is filled or emptied. In measuring such flows there are periods when the conduit may be empty of fluid, partially full or completely full. The flow may be free flowing (propelled only by the force of gravity). It may be constrained by an obstruction or other such downstream constraint. It may be flowing downstream due to an upstream pressure head, or it maybe be flowing upstream (in a reverse direction) owing to a downstream pressure head.
There is a class of flow meters that rely on primary devices. These systems require either a) the construction of flumes, weirs or other structures in the manhole or b) the installation and proper alignment of these structures in the manhole. While this is a reasonable approach to consider for sewage treatment plants where existing piping systems and structures can be designed and built around the needs of the primary device, it is typically impractical, expensive or simply not possible to properly install such structures in the sewer collection system where the monitoring point in question is deep underground.
Another class of meters utilizes an underwater velocity sensor and depth sensor installed in a pipe. The depth sensor can be installed above the flow or in the flow with the velocity sensor. Examples of this class include Petroff U.S. Pat. No. 5,020,374, Petroff U.S. Pat. No. 5,333,508, Nabity et al, U.S. Pat. No. 5,371,686, Petroff U.S. Pat. No. 7,672,797. Also Marsh U.S. Pat. No. 4,083,246 and Cushing U.S. Pat. No. 5,467,650.
A third class of devices uses a downward looking velocity sensor and a downward looking ultrasonic depth sensor to measure flow in the manhole (Marsh U.S. Pat. Nos. 5,684,250 and 5,811,688) or as it enters the manhole or over a flume (Bailey U.S. Pat. No. 5,315,880). The primary advantage of this approach being that it minimizes entry into confined space.
Accordingly, there is a need for a flow meter that measures depth and velocity with improved accuracy. All of the above referenced patent documents are incorporated herein by reference.